1. Field of the Invention
The present invention relates to a magnetic head that can record or reproduce in a plurality of track formats having different average track pitches.
2. Description of the Prior Art
Recently, there has been increasing requirement for magnetic disk drives for high density recording that can be used as external storages or image recording devices of a computer.
Floppy disk drives which are extensively used as external memories increase their recording density, for example, by shortening the recording wavelength so as to increase the line recording density.
Today, small diameter, metal coated video floppy disks are used in electronic still video devices and in data recording devices, which use an identical record format.
FIG. 1 is a schematic cross sectional view showing a head contacting mechanism of a conventional video floppy disk drive. In this figure, a video floppy disk 10 rotates in the X direction at a high speed of 3600 rpm, and hence, sliders commonly employed for supporting the floppy disk 10 at both surfaces thereof are not used. Instead, a head 8 is pressed in the Z direction by using air pressure produced by a backing plate 9 and, with the elasticity of the floppy disk 10, a good head contact can be achieved.
As another method, a negative pressure pad method is also adopted utilizing a negative pressure, which is produced by a backing plate provided at the same side as a head with respect to a floppy disk, in order to press the floppy disk to the head.
In either method, the area of a sliding portion at the tip of a head is not more than 100,000 .mu.m.sup.2 (for example, 300 .mu.m.times.300 or less), which is smaller than that of other types of floppy disk heads.
The current recording format of floppy disks for electronic still video apparatuses is specified as follows: the track pitch is 100 .mu.m; the track width is 60 .mu.m; the number of tracks concentrically formed on a floppy disk is 50; and each track can store one field.
Two types of recording heads are available for such recording and reproducing apparatuses: a 1-track video head; and a 2-track video head. The 1-track video head includes one magnetic core whose track width is 60 .mu.m, and whose sliding width on a floppy disk is less than 100 .mu.m in the track width direction, so that 1-field image or one track can be recorded or reproduced without moving the head. On the other hand, the 2-track video head includes two magnetic cores which are separated 100 .mu.m apart, each having a track width of 60 .mu.m, so that 1-frame image or two tracks can be simultaneously recorded or reproduced withou moving the head.
FIG. 2 is a view showing an arrangement of a conventional thin film head as an example of the 2-track video head. This head is fabricated as follows:
First, a bottom magnetic layer made of a soft magnetic thin film is deposited on a substrate 11, following by an insulating layer and coils 38. Subsequently, a nonmagnetic layer to be formed into magnetic gaps 34 are formed near a sliding surface, and then magnetic cores 32 making up top layers are formed in such a manner that the top layers are connected with the bottom layer through magnetic contact holes 36.
In FIG. 2, the end to end length l2 of the two tracks is specified at a rather small amount of 160 .mu.m. The length, however, is not short enough to achieve good contact between the sliding surface of the head and a floppy disk when compared with a 1-track head that performs recording to and reproducing from a 60 .mu.m wide track. It is preferable that the length l2 be made as short as possible.
On the other hand, in floppy disk drives extensively used as external storage with computers, efforts for improving track density are made so as to increase memory capacity by using a high output metal as a coating material.
In such a case, there arises a new problem that a disk drive for a high track density (i.e., a narrow track pitch) floppy disk must be capable of recording to and reproducing from a conventional low track density (i.e., a wide track pitch) floppy disk. In other words, compatibility of recording and reproducing with a low-level format is required.
FIG. 3 is a schematic bottom view showing a sliding surface of a conventional magnetic head used for downward compatibility, which is disclosed in Technical Research Report MR90-33 of The Institute of Electronics, Information and Communication Engineers of Japan.
This head comprises a slider 1, a high-level core 2 for recording and reproducing in a high-level format, and a low-level core 3 for recording and reproducing in a low-level format, thereby forming a 2-track composite head. The head further comprises a low-level magnetic gap 5 for recording and reproducing in a low-level format, and a high-level magnetic gap 4 which has a narrower track width than the low-level magnetic gap 5 so as to achieve high track density.
In addition, since the distance l1 between a track center of the high-level core 2 and a track center of the low-level core 3 is narrow, on the order of about 560 .mu.m, there arises crosstalk between the cores 2 and 3 owing to flux leakage. Accordingly, a magnetic shield material 6 is inserted between the cores 2 and 3. Furthermore, it has been attempted to reduce the crosstalk by variously modifying the 3-dimensional geometry of the cores, though not shown in FIG. 3.
FIG. 4 is a diagram for explaining a technique whereby a disk drive for driving a double track density floppy disk is made compatible with a low-level floppy disk. The technique is disclosed in Japanese Patent Application Laying-open No. 28,913/1987.
This head comprises a slider 1, a high-level core 2 for recording and reproducing in a high-level format, and a low-level core 3 for recording and reproducing in a low-level format, thereby forming a 2-track composite head. The recording pattern of the high-level format corresponds to a double track density or a half track pitch of a conventional one. The interval l1 between a track center of the high-level core 2 and a track center of the low-level core 3 is equal to the low-level track pitch of 500 .mu.m in this example.
The related arts described above present the following problems:
(1) As described above, one video floppy can contain only 25 frame images, and hence, an increase in storing capacity has been required.
(2) There is a great possibility that increasing storing capacity will be achieved by narrowing track pitch owing to recent developments in medium technique. However, since the current still video standard has been extensively used, a new machine must be compatible with the current format.
(3) In a downward compatible composite head incorporating a high-level core and a low-level core, the track pitch of at least 500 .mu.m is necessary to prevent crosstalk.
(4) In a floppy disk drive, the magnetic gap positions and azimuth angulars of a composite head as shown in FIG. 3 must be precisely controlled because a floppy disk is freely replaced. These requirements, however, decrease the efficiency of mass-production of composite heads, which in turn, increases the cost of composite heads. In particular, since a two track head is expensive in comparison with a single track head because the two track head includes twice as many assembly components like cores and coils as the single track head, the cost problem becomes more important.
(5) A 2-track composite head for a frame image has a sliding width with a floppy disk twice as wide as that of a 1-track head for a field image. For this reason, it is difficult for the two magnetic gaps of a composite magnetic head to simultaneously keep good contact with a floppy disk, thus decreasing reliability. Thus, a 2-track composite head whose track pitch is more than 500 .mu.m cannot achieve a good head contact with a small-diameter recording medium like a video floppy which rotates at a high speed. In addition, adjustment of head contact during mass production takes a long time, and this will increase cost of the 2-track composite heads.
(6) To further shorten the recording wavelength, the rail width l2 (see, FIG. 3) of the slider 1 must be reduced so that the gap amount between the floating slider 1 and the surface of a floppy disk is limited. In addition, to ensure reliable sliding of the head while maintaining a small gap amount when the disk driving speed is large to accomplish a high transfer rate, dimensions of the slider must be small: the track pitch l1 must be set smaller than conventional values of 500-600 .mu.m. This increases the crosstalk between the cores, and so, makes it difficult to maintain a certain signal quality. Moreover, space for providing a magnetic shield becomes very small, and so it becomes difficult to take effective steps for preventing crosstalk.